Homoarginine labeling is suitable for determination of protein absorption in miniature pigs

Intestinal proteolysis and absorption of dietary protein was followed using a chemical label of the lysine side-chain. Protein-bound lysine was transformed into homoarginine (HA), an amino acid that is not used for protein synthesis. Disappearance in the intestinal tract of HA originating from labeled casein and soybean isolate and appearance in peripheral blood was followed in four miniature pigs fitted with permanent T-cannulas at the proximal jejunum and ileum. Less than 0.2% of HA appeared in the digesta at the ileum when up to 13.5 mmol of HA was infused either into the jejunum or jugular vein. Several factors suggest that HA in fact traces the exogenous (dietary) protein: 1) the quick postprandial appearance of HA in plasma; 2) the high (greater than 97%) oro-ileal absorption of HA-labeled casein and soybean isolate; 3) the similarity of chymotryptic proteolysis in vitro of guanidinated and native casein; 4) the fact that only trace amounts of the marker reenter the intestinal lumen from the blood. Therefore HA-label is suitable for the differentiation of exogenous and endogenous protein in the chyme and thus the measurement of oro-ileal protein absorption and irreversible loss of endogenous protein in the chyme of the small intestine.